It has been known that, in a film in which layers having different refractive indices are alternately stacked, the film can be designed so as to reflect the regions from visible light to infrared ray by adjusting the optical film thickness.
Patent Literature 1 discloses that, using an alternately-stacked unit formed of a high-refractive-index layer A (refractive index: nA) and a low-refractive-index layer C (refractive index: nC), an infrared reflective film suppressing reflectivity unevenness in the visible right region can be attained by forming an alternately-stacked unit by alternately stacking a layer B having a different refractive index to satisfy a relationship in refractive indices of nB=√nAnC, together with the above-mentioned high-refractive-index layer A and low-refractive-index layer C, in the order of ABCB, and further presetting the ratio of the optical film thicknesses of the respective layers so as to be A 1/3, B 1/6 and C 1/3.
However, the above-mentioned constitution is designed in light of the performance for vertically-entered light beam, and significant reflectivity unevenness generates for beams with large incident angles. This also shows that the performance significantly decreases in the variation in the layer film thickness, which occurs during the production. Since it is necessary to stack three kinds of materials in sequence, the cost is higher than that of an alternately-stacked film having a two-layer constitution.
As materials having different refractive indices, inorganic materials have been conventionally used. Therefore, in order to form thin layers having different refractive indices, a deposition process in which such inorganic materials are deposited by physical deposition or chemical deposition is used. However, in these deposition processes, the apparatuses are huge and the formation of thin films requires high costs. These processes are not suitable for the formation of thin films having large surface areas. Therefore, an optical film to which a production process such as an application process that can form thin layers with large surface areas at lower costs without using a deposition process, can be applied is required.